Linear regulating valves of seat-type are generally used. One of their disadvantages is the leakage of the packing between the valve body and the spindle that moves the closure member. The linear movement of the spindle "pumps" medium through the packing, but at the same time it also causes wear grooves to the packings, parallel with the movement. These wear grooves form for the medium the shortest possible way out, so that a leakage appears even in the rest position of the spindle. Because of the wearing, the packing must be lengthened and more often tightened. A long packing increases the friction, which deteriorates the sensitivity and accuracy of the controlling.
In a tight rotating valve like a ball or plug valve used for controlling, significant problems are caused by the frictional forces. The friction between the closure sealing and the closure member is significant for the adjustment, as the moment arm of the friction force is big. Structures that aim at minimizing this problem are disclosed in patent publications U.S. Pat. Nos. 5,305,987 and 4,822,000 where an eccentric segment is used. Another way of solving the problem is to separate the seal from the closure member either mechanically (patent publication U.S. Pat. No. 4,867,414) or by means of a pressure difference (patent publication U.S. Pat. No. 4,747,578).
An other friction source in a rotating valve is the pressure difference of the inflow and outflow sides in the controlling position. This pressure difference generates a force pushing the closure member, so that the closure member must be supported with bearings to the valve body. A friction moment against the rotating of the closure member is generated by this force, thus making it difficult to achieve exact adjustment.
A third friction source is the sealing between the valve body and the shaft of the closure member, e.g. a packing. This packing is necessary when the actuating device is arranged outside the body. Due to the friction forces of the closure sealing and the bearing, the turning shaft must be dimensioned against the torsion, so that the diameter of the packing becomes bigger than the one of a seat valve of corresponding size. The moment arm of the friction force generated by the seal forces of the shaft grows bigger.
When the turning shaft extends outside the body, an axial pressure force is generated which must be supported either with arrangements inside or outside the body. This supporting generates a friction moment.
In addition to friction moments, the rotating valves also have a significant dynamic moment that is caused by the ununiformly distributed speed increase in the flow channel of the closure member or of the body, and by the resulting pressure difference. This moment disturbs the exact controlling, because its magnitude depends on the angle of opening and it is not linear.
Still one disadvantage of the rotating valves is the noise created in connection with the controlling. A ball valve and its derivatives have a big flow capacity but one drawback is the noise created by cavitation. The cavitation bubbles collapse outside the valve in the piping near the pipe wall, from where the noise is transmitted to the neighbourhood. This noise has been decreased efficiently by arranging the pressure reduction in multiple stages (see patent publication U.S. Pat. No. 4,530,375).
Patent publication U.S. Pat. No. 4,149,563 discloses a so called silent seat-valve, where the cavitation bubbles collapse inside a liquid in an isolated space.
Patent publication U.S. Pat. No. 4,651,775 discloses a three-way valve, the closure member of which is hollow and perforated. Patent publication U.S. Pat. No. 4,778,152 discloses a plug valve, the plug of which is equipped with a transverse opening. From patent publication U.S. Pat. No. 5,219,148 there is known a valve having a plate-like closure member arranged transversely with respect to the turning shaft, and this plate has several openings.